Light emitting diodes (LEDs) have been gaining wide spread applications in liquid display, signage and general-purpose lightings due to the rapid progress in the solid-state lighting technology. Compared with existing conventional lighting sources such as incandescent lamps and fluorescent lamps, LEDs have relatively longer operational lifetime in the range of 80,000-100,000 hours attributing to no high-field sputtering of filament. LEDs available in the market are now encapsulated with less glass, which significantly improves their reliability and safety to the handler. Free of toxic mercury, LED can be disposed safely at the end of its lifetime. Other advantageous features such as flicker free, smooth dimming, low-voltage operation and good color rendering property make LED an emerging technology that may dominate the lighting market in the near future.
The general photo-electro-thermal (PET) theory points out that the device level multichip design with low-power chips offers advantageous features over single-chip high-power design in terms of higher efficacy and lower junction temperature. Similarly, on the system level, a distributed LED system based on a plurality of relatively low-power LEDs can have similar advantages over a concentrated system consisting of a small number of high-power LEDs for the same system power. Since LEDs are current-driven devices and its luminous intensity is directly related to the forward current applied, when driving multiple LEDs, a series connection structure is superior to a parallel one because all LEDs in the series string can operate at the same current without current sharing and chromaticity variation issues. However, the number of LEDs connected in series is highly limited by the output voltage provided by the power supply and therefore the use of parallel LED strings has been acrimony practice particularly for high power applications (say >25 W). Such parallel LED strings arrangement leads to current imbalance issue because of the manufacturing tolerance, aging and temperature variations in LEDs, resulting in variations in the luminous intensity and color. Furthermore, one or more LED strings may exceed its absolute maximum rating current even though the average current of each LED string is less than the rating current when parallel LED strings are used without current sharing means.
There are several current sharing methods for driving multistring LEDs connected in parallel. A straightforward approach is to add a ballast resistor in series with each LED string to minimize current differences. This approach is very simple; however, it suffers from poor operating efficiency due to the significant power losses dissipated on the added ballast resistors. A lossless capacitor can be used to replace the loss ballast resistor to reduce the unnecessary loss when the LEDs are driven with AC source or coupled with rectifier. The main drawback of these methods is that the forward current of each LED string cannot be controlled precisely. Currently, a linear current regulator for each string has been employed to ensure good current sharing effect, at the expense of considerable power loss on the current regulator. Another approach is to set up a separate voltage source for each LEDs string. A modular power converter architecture based on parallel or series input connected converters with separate LED string loads can be used. Each LED string current is independently sensed and controlled to follow the same reference. Without loss ballast resistor or linear current regulator, the two LED driver architectures have relatively higher conversion efficiency. However, the architectures are complex and expensive because each LED string needs a set of main circuit and controller.